Semiconductor light emitting element and method for manufacturing the same

ABSTRACT

The semiconductor light emitting element is a semiconductor light emitting element comprising a semiconductor layer including a light emitting layer, wherein a surface of the semiconductor light emitting element includes a light extraction surface. At least one of the light extraction surface and an interface between two layers having different refractive indexes in the semiconductor light emitting element is provided with a periodic recessed and projecting structure having a period that exceeds 0.5 times as great as a wavelength of light emitted from the light emitting layer, and a minute recessed and projecting structure located on a surface of the periodic recessed and projecting structure and having an average diameter that is not more than 0.5 times as great as the wavelength of the light.

TECHNICAL FIELD

The present invention relates to a semiconductor light emitting elementsuch as a light emitting diode (LED), and particularly to asemiconductor light emitting element improved in light extraction of thelight emitted inside the element to the outside, and a method formanufacturing the semiconductor light emitting element.

BACKGROUND ART

A semiconductor light emitting element is formed of several layers suchas a light emitting layer, an n-type semiconductor layer, a p-typesemiconductor layer, an electrode layer, and a supporting substrate.Therefore, the light emitted in the light emitting layer inside thesemiconductor element is extracted to the outside after having passedthrough these several layers. However, when the light passes through aboundary between media having different refractive indexes, i.e., aninterface between layers or a surface, a certain rate of reflectionoccurs inevitably. In addition, when the light passes through or isreflected from a medium layer having an absorption coefficient for awavelength (emission wavelength) of the aforementioned light, a certainrate of light absorption occurs. Therefore, it is generally difficult toefficiently extract the light emitted in the light emitting layer to theoutside of the semiconductor light emitting element.

Particularly when the light travels from a medium having a highrefractive index to a medium having a low refractive index, totalreflection of the light occurs and the light having an angle equal to orlarger than a critical angle cannot be extracted to the outside. On thesurface of the semiconductor light emitting element, i.e., an interfacebetween the air (or a sealing material) and the semiconductor element, adifference in refractive index between the media is large, and thus, thecritical angle at which total reflection occurs becomes small, whichresults in an increase in ratio of the light totally reflected from theinterface.

For example, a refractive index n of a sapphire substrate is 1.8 and acritical angle to the air is 33.7 degrees. Namely, when a sapphiresubstrate is used as the substrate forming the semiconductor lightemitting element and when the light is extracted through the sapphiresubstrate to the air side, the light having an incidence angle largerthan 33.7 degrees is totally reflected and cannot be extracted to theoutside. In the case of an aluminum nitride (AlN) substrate having alarger refractive index (refractive index n=2.29), a critical angle is25.9 degrees and only a smaller amount of the light can be extracted tothe outside.

By using theoretical calculation of the light radiation propagationproperty with a finite-difference time-domain method (FDTD method), thelight extraction efficiency in a semiconductor light emitting elementhaving an AlGaN layer stacked on an AlN substrate was calculated, forexample. As a result, considering absorption or the like by a p-type GaNlayer located opposite to the AlN substrate as seen from a lightemitting portion inside the AlGaN layer, the extraction efficiency ofthe light that can be extracted from the surface (light extractionsurface) side of the AlN substrate, of the light with a wavelength of265 nm radiated from the light emitting portion, is extremely low, i.e.,about 4%.

In order to deal with the aforementioned problem, there has beenproposed a semiconductor light emitting element having a nanometer-scalerecessed and projecting structure on a substrate surface (lightextraction surface). For example, PTD 1 discloses that a lightextraction surface is provided with a recessed and projecting structurehaving an average period that is not more than twice as great as anaverage optical wavelength of the light emitted from a light emittinglayer. PTD 1 proposes a method for reducing a ratio of the light totallyreflected from the light extraction surface (i.e., suppressingreflection of the light from the element surface), by forming theaforementioned recessed and projecting configuration. However, it is noteasy to form the nanometer-scale recessed and projecting structure onthe surface of the semiconductor light emitting element. In addition,the light extraction efficiency greatly varies depending on the shape ofthe recessed and projecting structure and the emission wavelength, andthus, the effect is not sufficiently obtained.

As the emission wavelength becomes shorter, the required period of therecessed and projecting structure (e.g., in the case of a projectingstructure, a distance between a vertex portion of a projecting structureand a vertex portion of an adjacent projecting structure) becomesshorter, and thus, fabrication of the recessed and projecting structurebecomes difficult. Particularly in a semiconductor light emittingelement that emits the light in an ultraviolet or deep ultravioletwavelength range, it is difficult to fabricate the recessed andprojecting structure of such size by optical lithography. As a result,problems such as an increase in fabrication cost and a reduction inyield and productivity arise, and thus, fabrication of such recessed andprojecting structure is not practical.

PTD 1 (Japanese Patent Laying-Open No. 2005-354020) discloses a methodfor heating and flocculating a deposited metal to form a nanometer-sizedminute metal mask, forming the nanometer-sized minute metal mask on thelight extraction surface, and etching a surface of the light extractionsurface, in order to form the nanometer-scale periodic recessed andprojecting structure. However, in the case of the aforementionedperiodic mask using the flocculation effect, arrangement of the recessedand projecting structure is random and the non-uniformity of a shapethereof is high. Therefore, variations in power of the light output fromthe semiconductor light emitting element to the outside are large and itis difficult to provide a semiconductor light emitting element thatemits the stable and uniform light.

NPD 1 (ISDRS 2011, Dec. 7-9, 2011, College Park, Md., USA, WP2-04)discloses a method for roughening a substrate surface by wet etching inorder to form a nanometer-scale recessed and projecting structure.However, the recessed and projecting structure formed by a method usingwet etching also has a random structure having a non-uniform shape.Therefore, the light extraction efficiency greatly varies and the effectof enhancing the light extraction efficiency is also insufficient.

According to NPD 2 (Appl. Phys. Express 3 (2010) 061004), in asemiconductor light emitting element that emits the deep ultravioletlight, a surface periodic recessed and projecting structure is providedby lithography and dry etching. However, a period of the recessed andprojecting structure is 500 nm, which is approximately twice as great asan emission wavelength, and the effect of enhancing the light extractionefficiency is not sufficiently obtained. In addition, variations inlight output are extremely large.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2005-354020

Non Patent Document

-   NPD 1: ISDRS 2011, Dec. 7-9, 2011, College Park, Md., USA, WP2-04-   NPD 2: Appl. Phys. Express 3 (2010) 061004

SUMMARY OF INVENTION Technical Problem

As described above, the semiconductor light emitting element having thenanometer-scale recessed and projecting structure on the substratesurface (light extraction surface) has been proposed to enhance thelight extraction efficiency. However, in such conventional lightemitting element, optimum values of the period of the recessed andprojecting structure, the height and shape of the projection forming therecessed and projecting structure, and the like are uncertain and varydepending on the emission wavelength, the refractive index of thesubstrate, and the like. Therefore, the sufficient effect is notproduced under the present circumstances. Furthermore, as the emissionwavelength becomes shorter, a smaller-scale recessed and projectingstructure needs to be formed on the substrate surface (light extractionsurface), and thus, fabrication of the recessed and projecting structurebecomes more difficult. Therefore, a great challenge is to reproduciblyand uniformly form the recessed and projecting structure thatsufficiently enhances the light extraction efficiency, and further tomake uniform and stable the power of the light output from thesemiconductor light emitting element to the outside, even if theemission wavelength is short.

Accordingly, an object of the present invention is to solve theconventional problem described above, and to provide a semiconductorlight emitting element that achieves high light extraction efficiencyand uniform light output even if an emission wavelength is short, and amethod for manufacturing the semiconductor light emitting element bywhich the semiconductor light emitting element can be manufactured withhigh reproducibility and high productivity.

Solution to Problem

A semiconductor light emitting element according to the presentinvention is a semiconductor light emitting element comprising asemiconductor layer including a light emitting layer, wherein a surfaceof the semiconductor light emitting element includes a light extractionsurface. At least one of the light extraction surface and an interfacebetween two layers having different refractive indexes in thesemiconductor light emitting element is provided with a periodicrecessed and projecting structure having a period that exceeds 0.5 timesas great as a wavelength of light emitted from the light emitting layer,and a minute recessed and projecting structure located on a surface ofthe periodic recessed and projecting structure and having an averagediameter that is not more than 0.5 times as great as the wavelength ofthe light.

Advantageous Effects of Invention

According to the present invention, the semiconductor light emittingelement that achieves high light extraction efficiency can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a first embodiment of asemiconductor light emitting element according to the present invention.

FIG. 2 is a schematic plan view of a light extraction surface of thesemiconductor light emitting element shown in FIG. 1.

FIG. 3 is a schematic partial cross-sectional view taken along line inFIG. 2.

FIG. 4 is a schematic plan view for describing a modification of thesemiconductor light emitting element shown in FIG. 1.

FIG. 5 is a flowchart for describing a method for manufacturing thesemiconductor light emitting element shown in FIG. 1.

FIG. 6 is a schematic plan view for describing a second embodiment ofthe semiconductor light emitting element according to the presentinvention.

FIG. 7 is a schematic partial cross-sectional view taken along lineVII-VII in FIG. 6.

FIG. 8 is a schematic plan view for describing a modification of thesemiconductor light emitting element shown in FIG. 6.

FIG. 9 is a flowchart for describing a method for manufacturing thesemiconductor light emitting element shown in FIG. 6.

FIG. 10 is a schematic plan view of a semiconductor light emittingelement used as a sample of Example 1.

FIG. 11 is a schematic cross-sectional view taken along line XI-XI inFIG. 10.

FIG. 12 is a scanning electron microscope photograph of a lightextraction surface of the semiconductor light emitting element shown inFIG. 10.

FIG. 13 is a scanning electron microscope photograph of the lightextraction surface of the semiconductor light emitting element shown inFIG. 10.

FIG. 14 is a graph showing an experimental result in Example 1.

FIG. 15 is a scanning electron microscope photograph of a lightextraction surface of a semiconductor light emitting element used as asample of Example 2.

FIG. 16 is a scanning electron microscope photograph of the lightextraction surface of the semiconductor light emitting element used asthe sample of Example 2.

FIG. 17 is a scanning electron microscope photograph of the lightextraction surface of the semiconductor light emitting element used asthe sample of Example 2.

FIG. 18 is a graph showing an experimental result in Example 2.

FIG. 19 is a graph showing a simulation calculation result in Example 3.

FIG. 20 is a graph showing a simulation calculation result in Example 3.

FIG. 21 is a graph showing a simulation calculation result in Example 4.

FIG. 22 is a graph showing a simulation calculation result in Example 4.

FIG. 23 is a graph showing an experimental result in Example 5.

FIG. 24 is a graph showing an experimental result in Example 6.

FIG. 25 is a graph showing an experimental result in Example 7.

FIG. 26 is a graph showing experimental results in Examples 8 and 9.

FIG. 27 is a graph showing experimental values and a simulationcalculation result in Examples.

DESCRIPTION OF EMBODIMENTS

First, an overview of an embodiment will be described.

(1) A semiconductor light emitting element according to the presentembodiment is a semiconductor light emitting element comprising asemiconductor layer including a light emitting layer (active layer 13),wherein a surface of the semiconductor light emitting element includes alight extraction surface, and at least one of the light extractionsurface and an interface between two layers having different refractiveindexes in the semiconductor light emitting element is provided with aperiodic recessed and projecting structure 21 having a period thatexceeds 0.5 times as great as a wavelength of light emitted from thelight emitting layer, and a minute recessed and projecting structure 22located on a surface of periodic recessed and projecting structure 21and having an average diameter that is not more than 0.5 times as greatas the wavelength of the light.

With such a configuration, periodic recessed and projecting structure 21having the period corresponding to the wavelength (emission wavelength)of the light emitted from the light emitting layer and minute recessedand projecting structure 22 having the average diameter corresponding tothis wavelength are formed on the light extraction surface. Therefore,the light extraction efficiency can be reliably enhanced as comparedwith the case of not having these recessed and projecting structures onthe light extraction surface. Namely, even when a periodic recessed andprojecting structure having a period greater than the wavelength isformed, the light extraction efficiency can be sufficiently enhanced bycombining the periodic recessed and projecting structure with a minuterecessed and projecting structure. Furthermore, when the emissionwavelength is short (e.g., equal to or shorter than 450 nm, or equal toor shorter than 350 nm), the effect of suppressing an increase in costrelated to manufacturing of the periodic recessed and projectingstructure is remarkable in the semiconductor light emitting elementaccording to the present embodiment. In addition, since the periodicrecessed and projecting structure can be formed with the period greaterthan the emission wavelength, formation of the uniform recessed andprojecting structure is easy.

(2) In the aforementioned semiconductor light emitting element, anarrangement pattern of periodic recessed and projecting structure 21 maybe a triangular lattice-like pattern. In this case, the number per unitarea of projecting portions of periodic recessed and projectingstructure 21 can be easily increased.

(3) In the aforementioned semiconductor light emitting element, periodicrecessed and projecting structure 21 may include a projection(projecting portion) made of a high refractive index material having arefractive index higher than that of the air.

A cross-sectional area of the projection at a surface perpendicular to adirection toward the light extraction surface (rear surface 16A ofsubstrate 16) from the light emitting layer (active layer 13) may becomesmaller with increasing distance from the light emitting layer (activelayer 13). With such a configuration, when an external medium to thelight extraction surface is the air, the efficiency of light extractionfrom the light extraction surface can be reliably enhanced.

(4) In the aforementioned semiconductor light emitting element, theprojection may have a conical shape or a semi-elliptical sphericalshape. In this case, the projection can be easily formed by using arelatively common process such as etching.

(5) In the aforementioned semiconductor light emitting element, thelight emitting layer may include a group-III nitride semiconductor. Thesemiconductor layer may include: an n-type group-III nitridesemiconductor layer (n-type semiconductor layer 15) having an n-typeconductivity; and a p-type group-III nitride semiconductor layer (p-typesemiconductor layer 12) located opposite to the n-type group-III nitridesemiconductor layer as seen from the light emitting layer and having ap-type conductivity. By using the aforementioned group-III nitridesemiconductor, the semiconductor light emitting element that emits theshort-wavelength light having an emission wavelength equal to or shorterthan 450 nm can be obtained.

(6) The aforementioned semiconductor light emitting element may furtherinclude a transparent substrate arranged on the light extraction surfaceside as seen from the light emitting layer and having transparency tothe light emitted from the light emitting layer. In this case, the rearsurface (rear surface opposite to a main surface having thesemiconductor layer formed thereon) of the transparent substrate can beused as the light extraction surface.

(7) In the aforementioned semiconductor light emitting element, thetransparent substrate may be an aluminum nitride substrate. In thiscase, a defect density of the semiconductor layer including the lightemitting layer formed of the group-III nitride semiconductor can besignificantly reduced.

(8) In the aforementioned semiconductor light emitting element, thewavelength of the light emitted from the light emitting layer may beequal to or shorter than 450 nm. When the emission wavelength is shortas described above, the aforementioned effect can be remarkablyobtained.

(9) In the aforementioned semiconductor light emitting element, a heightH1 of periodic recessed and projecting structure 21 may be not less than⅓ times and not more than 5 times as great as period L1 of periodicrecessed and projecting structure 21, and an average height of minuterecessed and projecting structure 22 may be not less than 0.1 times andnot more than 10 times as great as the average diameter of minuterecessed and projecting structure 22. In this case, the light extractionefficiency can be reliably enhanced.

(10) A semiconductor light emitting element according to the presentembodiment includes: a substrate 16 made of aluminum nitride; and asemiconductor layer formed on a main surface of substrate 16. Thesemiconductor layer includes: a light emitting layer (active layer 13)including a group-III nitride semiconductor; and an n-type group-IIInitride semiconductor layer (n-type semiconductor layer 15) having ann-type conductivity and a p-type group-III nitride semiconductor layer(p-type semiconductor layer 12) having a p-type conductivity, the n-typegroup-III nitride semiconductor layer and the p-type group-III nitridesemiconductor layer being arranged to sandwich the light emitting layer.A wavelength of light emitted from the light emitting layer is equal toor shorter than 350 nm. A rear surface of substrate 16 located oppositeto the main surface is provided with a periodic recessed and projectingstructure 21 having a period that is not less than ⅓ times and not morethan 5 times as great as a value (evaluated value) obtained by dividingthe wavelength of the light emitted from the light emitting layer by adifference between a refractive index of the aluminum nitride formingthe substrate and a refractive index of an external medium locatedoutside the substrate.

In this case, the period of periodic recessed and projecting structure21 is determined in accordance with the wavelength (emission wavelength)of the light emitted from the light emitting layer, the refractive indexof substrate 16 made of AlN, and the refractive index of the externalmedium. Therefore, the efficiency of light extraction from the rearsurface (light extraction surface) of substrate 16 can be reliablyenhanced.

The aforementioned period is preferably not less than 0.5 times and notmore than 4 times, and more preferably not less than 1 time and not morethan 3 times as great as the evaluated value.

(11) In the aforementioned semiconductor light emitting element, anarrangement pattern of periodic recessed and projecting structure 21 maybe a triangular lattice-like pattern. In this case, the number per unitarea of projecting portions of periodic recessed and projectingstructure 21 can be easily increased.

(12) In the aforementioned semiconductor light emitting element, theperiodic recessed and projecting structure may include a projection, andthe projection may have a conical shape or a semi-elliptical sphericalshape. In this case, the projection can be easily formed by using arelatively common process such as etching.

(13) In the aforementioned semiconductor light emitting element, aheight of the periodic recessed and projecting structure may be not lessthan ⅓ times and not more than 5 times as great as the period of theperiodic recessed and projecting structure. In this case, the lightextraction efficiency can be reliably enhanced. The aforementionedheight is preferably not less than 0.5 times and not more than twice,and more preferably not less than 0.6 times and not more than 1.8 timesas great as the evaluated value.

(14) A method for manufacturing a semiconductor light emitting elementaccording to the present embodiment includes the steps of: preparing anelement member that should form a semiconductor light emitting elementincluding a semiconductor layer having a light emitting layer; forming amask layer having a pattern on a region of the element member thatshould form a light extraction surface of the semiconductor lightemitting element; and forming a periodic recessed and projectingstructure by partially removing the region that should form the lightextraction surface by etching using the mask layer as a mask. The masklayer is a metal mask layer. In the step of forming a periodic recessedand projecting structure, by performing dry etching using afluorine-based gas as an etching gas, the periodic recessed andprojecting structure is formed and a minute recessed and projectingstructure is formed on a surface of the periodic recessed and projectingstructure in a step of removing a residue of the mask layer. Theperiodic recessed and projecting structure has a period that exceeds 0.5times as great as a wavelength of light emitted from the light emittinglayer. The minute recessed and projecting structure has an averagediameter that is not more than 0.5 times as great as the wavelength ofthe light. In addition, the periodic recessed and projecting structureand the minute recessed and projecting structure are characterized bybeing made of the same material. With such a configuration, thesemiconductor light emitting element according to the present embodimentcan be easily obtained.

(15) A method for manufacturing a semiconductor light emitting elementaccording to the present embodiment includes the steps of: preparing asubstrate made of aluminum nitride, and an element member that is formedon a main surface of the substrate and should form a semiconductor lightemitting element including a semiconductor layer having a light emittinglayer; forming a mask layer having a pattern on a region of the elementmember that should form a light extraction surface of the semiconductorlight emitting element; and forming a periodic recessed and projectingstructure by partially removing the region that should form the lightextraction surface by etching using the mask layer as a mask. Theperiodic recessed and projecting structure has a period that is not lessthan ⅓ times and not more than 5 times as great as a value obtained bydividing a wavelength of light emitted from the light emitting layer bya difference between a refractive index of the aluminum nitride formingthe substrate and a refractive index of an external medium locatedoutside the substrate. With such a configuration, the semiconductorlight emitting element according to the present embodiment can be easilyobtained.

(16) In the aforementioned semiconductor light emitting element, theperiod of periodic recessed and projecting structure 21 may be not lessthan 1 time as great as the wavelength of the light. In this case,periodic recessed and projecting structure 21 can be easilymanufactured.

(17) In the aforementioned semiconductor light emitting element, theperiod of periodic recessed and projecting structure 21 may be not lessthan twice as great as the wavelength of the light. In addition, anaverage diameter of minute recessed and projecting structure 22 may benot more than 0.4 times as great as the wavelength of the light. In thiscase, an increase in manufacturing cost of the semiconductor lightemitting element can be avoided and the light extraction efficiency canbe reliably enhanced.

(18) In the aforementioned semiconductor light emitting element, thetransparent substrate may be a sapphire substrate. With such aconfiguration as well, the semiconductor light emitting element withenhanced light extraction efficiency can be obtained.

(19) In the aforementioned semiconductor light emitting element, thewavelength (emission wavelength) of the light emitted from the lightemitting layer may be equal to or shorter than 350 nm. In this case, theeffect of the present embodiment is remarkable in the semiconductorlight emitting element that emits the short-wavelength light describedabove.

Next, a specific example of the embodiment of the present invention willbe described with reference to the drawings as appropriate. A lightemitting element described below is for embodying the technical idea ofthe present invention and does not limit the present invention to thefollowing. Particularly, dimensions, materials, shapes, and otherrelative arrangements of components described below are not intended tolimit the scope of the present invention to such values unless specifiedto the contrary, and are merely description examples. Sizes, positionalrelations and the like of members shown in each figure may beexaggerated to clarify the description. In addition, the elementsconstituting the present invention may be configured such that aplurality of elements are formed by the same member and this one memberserves as the plurality of elements, or conversely, may be configuredsuch that the function of one member is shared by a plurality ofmembers. Furthermore, in the below-described embodiments as well, theconfigurations and the like can be combined as appropriate and applied,unless otherwise excluded.

First Embodiment

FIGS. 1 to 3 conceptually show a structure of a semiconductor lightemitting element according to a first embodiment of the presentinvention. Referring to FIGS. 1 to 3, the semiconductor light emittingelement mainly includes a substrate 16 made of MN (aluminum nitride), ann-type semiconductor layer 15, an active layer 13, a p-typesemiconductor layer 12, a positive electrode 11, and a negativeelectrode 14. N-type semiconductor layer 15 is formed on a main surfaceof substrate 16. A projection is formed on a part of a surface of n-typesemiconductor layer 15, and active layer 13 is formed on thisprojection. P-type semiconductor layer 12 is formed on active layer 13.Positive electrode 11 is formed on p-type semiconductor layer 12. Inaddition, negative electrode 14 is formed in a region on the surface ofn-type semiconductor layer 15 where the aforementioned projection is notformed.

A wavelength of the light emitted from active layer 13 serving as alight emitting layer is equal to or shorter than 350 nm. A rear surfaceof substrate 16 located opposite to the main surface is provided with aperiodic recessed and projecting structure 21 having a period L1 that isnot less than ⅓ times and not more than 5 times as great as a value(reference value) obtained by dividing the wavelength of the lightemitted from active layer 13 by a difference between a refractive indexof aluminum nitride forming substrate 16 and a refractive index of theair which is an external medium located outside substrate 16.

The components will be described below individually.

<Substrate>

A substrate which allows a nitride semiconductor crystal to beepitaxially grown on a surface thereof and which satisfies a conditionthat a transmittance is high with respect to the wavelength range of thelight emitted by the semiconductor light emitting element (e.g., thelight transmittance is equal to or higher than 50%) can be selected andused as substrate 16. Examples of a material of substrate 16 include AlNdescribed above, sapphire, GaN and the like.

As described above, substrate 16 is provided with periodic recessed andprojecting structure 21 on the light extraction surface (rear surface).Specifically, periodic recessed and projecting structure 21 includes aprojecting portion, and this projecting portion has a conical shape(e.g., a conical shape having a diameter D1 of a bottom surface, aheight H1 from the bottom surface to a vertex, and an angle θ formed bya side surface and the bottom surface) as shown in FIGS. 2 and 3. Theprojecting portion may have a semi-elliptical spherical shape as shownin FIG. 4.

Arrangement of the periodic recessed and projecting structure(arrangement of the projecting portion) may be periodic arrangement suchas triangular lattice arrangement, square lattice arrangement andhexagonal lattice arrangement, and is preferably triangular latticearrangement that achieves the maximum filling factor. Furthermore,periodic recessed and projecting structure 21 may have period L1 that isnot less than ⅓ times and not more than 5 times as great as the value(reference value) obtained by dividing the emission wavelength of thesemiconductor light emitting element by the difference between therefractive index of aluminum nitride forming substrate 16 and therefractive index of the air which is the external medium located outsidesubstrate 16. In addition, it is preferable that height H1 of periodicrecessed and projecting structure 21 is within a range of not less than⅓ times and not more than 5 times as great as period L1.

Period L1 of periodic recessed and projecting structure 21 describedabove is more preferably not less than 0.5 times and not more than 4times, and further preferably not less than 1 time and not more than 3times, as great as the aforementioned reference value. With such aconfiguration, the light extraction efficiency can be enhanced morereliably. In addition, height H1 of periodic recessed and projectingstructure 21 is preferably not less than 0.5 times and not more thantwice, and more preferably not less than 0.6 times and not more than 1.8times, as great as period L1. With such a configuration as well, thelight extraction efficiency can be enhanced more reliably.

Next, a method for fabricating periodic recessed and projectingstructure 21 will be described below. Periodic recessed and projectingstructure 21 can be fabricated by a process including firstly an etchingmask fabrication step (step (S41) in FIG. 5), secondly an etching step(step (S42) in FIG. 5), and thirdly a mask removal step (step (S43) inFIG. 5). The etching mask fabrication step is a step of fabricating anetching mask pattern on the rear surface of substrate 16, and anarbitrary method such as an electron beam lithography method, an opticallithography method and a nanoimprint lithography method can be applied.Alternatively, in order to enhance an etching selectivity in the etchingstep, a metal mask pattern may be fabricated by forming a mask pattern(e.g., resist mask) having a pattern by the aforementioned arbitrarymethod, and then, depositing a metal to cover the mask pattern, andthen, removing a part of the metal together with the mask pattern by aliftoff method.

By using the mask pattern as the etching mask, the rear surface ofsubstrate 16 is etched to form a desired pattern on the rear surface ofsubstrate 16. Dry etching such as inductively-coupled plasma (ICP)etching and reactive ion etching (RIE), wet etching using an acidsolution or an alkaline solution as an etchant, or the like can beapplied as a method for etching. In order to form a highly periodicpattern, it is preferable to apply dry etching. In the etching stepusing dry etching, a resin material such as a resist or a metal can beused as an etching mask, and a chlorine-based gas, a fluorine-based gas,a bromine-based gas or the like can be applied as an etching gas.Furthermore, a gas obtained by mixing hydrogen, oxygen or the like withthe aforementioned etching gas may be used.

After the aforementioned etching step, the mask removal step isperformed. Namely, a residue of the etching mask is removed. A methodfor removing the residue of the etching mask may be determined asappropriate, depending on a material of the etching mask. For example,when the etching mask is made of a metal, the residue may be removed byusing an acid solution or an alkaline solution having solubility withrespect to the metal.

A sealant such as resin, glass and quartz may be formed on periodicrecessed and projecting structure 21. A recessed and projectingstructure may be further formed on a surface of the sealant. Aconfiguration of the recessed and projecting structure may be similar tothe configuration of periodic recessed and projecting structure 21described above.

<Stacked Semiconductor Layer>

The stacked semiconductor layer is formed of a group-III nitridesemiconductor and is formed by stacking n-type semiconductor layer 15,active layer 13 and p-type semiconductor layer 12 on substrate 16 inthis order as shown in FIG. 1. The stacked semiconductor layer isstacked by using a method such as a metal organic chemical vapordeposition method (MOCVD method), a metal organic vapor phase epitaxymethod (MOVPE method), a molecular beam epitaxy method (MBE method), anda hydride vapor phase epitaxy method (HYPE method).

N-type Semiconductor Layer:

N-type semiconductor layer 15 is a semiconductor layer made ofAl_(x)In_(y)GaN_(z) (x, y and z are rational numbers satisfying 0<x≤1.0,0≤y≤0.1 and 0≤z≤1.0, and x+y+z=1.0), and preferably includes an n-typeimpurity. The impurity is not particularly limited, and examples of theimpurity include silicon (Si), germanium (Ge), tin (Sn) and the like,and the impurity is preferably Si or Ge. A concentration of the n-typeimpurity may be set to be not less than 1.0×10¹⁷/cm³ and not more than1.0×10²⁰/cm³. From the perspectives of the crystallinity and the contactproperty of n-type semiconductor layer 15, the concentration of then-type impurity is preferably not less than 1.0×10¹⁸/cm³ and not morethan 1.0×10¹⁹/cm³.

In addition, a layer thickness of n-type semiconductor layer 15 is notless than 100 nm and not more than 10000 nm. From the perspectives ofthe crystallinity and the conductivity of n-type semiconductor layer 15,the layer thickness of n-type semiconductor layer 15 is preferably notless than 500 nm and not more than 3000 nm.

Active Layer:

Active layer 13 has a multiquantum well structure. Active layer 13 has astacked structure formed by alternately stacking a well layer made ofAl_(x)In_(y)GaN_(z) (x, y and z are rational numbers satisfying 0<x≤1.0,0≤y≤0.1 and 0≤z<1.0, and x+y+z=1.0), and a barrier layer made ofAl_(x)In_(y)GaN_(z) (x, y and z are rational numbers satisfying 0<x≤1.0,0≤y≤0.1 and 0≤z<1.0, and x+y+z=1.0) having a bandgap energy greatherthan that of the well layer. A layer thickness of the well layer is notless than 1 nm, and preferably not less than 2 nm. A layer thickness ofthe barrier layer is not less than 1 nm, and preferably not less than 2nm.

P-type Semiconductor Layer:

P-type semiconductor layer 12 is formed of, for example, a p-type cladlayer and a p-type contact layer. The p-type clad layer is made ofAl_(x)in_(y)GaN_(z) (x, y and z are rational numbers satisfying 0<x≤1.0,0≤y≤0.1 and 0≤z<1.0, x+y+z=1.0). Since an electron needs to be confinedwithin active layer 13, it is preferable that the p-type clad layer hasa bandgap energy greater than that of the semiconductor layer formingactive layer 13. Therefore, it is preferable that the Al composition ofthe p-type type clad layer is greater than the Al composition of thesemiconductor layer forming active layer 13.

Examples of an impurity in the p-type clad layer suitably includemagnesium (Mg). A concentration (doping concentration) of Mg is not lessthan 1.0×10¹⁷/cm³, and preferably not less than 1.0×10¹⁷/cm³. A layerthickness of the p-type clad layer is not less than 5 nm and not morethan 1000 nm, and preferably not less than 10 nm and not more than 50nm.

The p-type contact layer is made of Al_(x)In_(y)GaN_(z) (x, y and z arerational numbers satisfying 0<x≤1.0, 0≤y≤0.1 and 0≤z<1.0, andx+y+z=1.0). It is preferable that the Al composition of the p-typecontact layer is smaller than the Al composition of the p-type cladlayer. A reason for this is that the excellent contact property iseasily obtained when the bandgap energy of the p-type contact layer issmaller than that of the p-type clad layer. Similarly to the p-type cladlayer, examples of an impurity in the p-type contact layer suitablyinclude Mg. A doping concentration of Mg can be set to be not less than1.0×10¹⁷/cm³. From the perspectives of the ultraviolet lighttransmission property and the contact property of the p-type contactlayer, a layer thickness of the p-type contact layer is not less than 1nm and not more than 50 nm, and preferably not less than 5 nm and notmore than 30 nm.

<Negative Electrode Layer>

Negative electrode 14 is formed on an exposed surface (upper surfacesurrounding the projection of n-type semiconductor layer 15) of n-typesemiconductor layer 15. The exposed surface of n-type semiconductorlayer 15 is formed by partially removing a part of n-type semiconductorlayer 15 as well as active layer 13 and p-type semiconductor layer 12(e.g., by etching and the like). Dry etching such as reactive ionetching and inductively-coupled plasma etching can be suitably used as amethod for etching. In order to remove a portion damaged by etching onthe etched surface (exposed surface) of n-type semiconductor layer 15,after the exposed surface of n-type semiconductor layer 15 is formed, itis preferable to perform surface treatment with an acid or alkalinesolution. Thereafter, negative electrode 14 having an ohmic property isformed on the aforementioned exposed surface of n-type semiconductorlayer 15.

Patterning of the electrodes such as negative electrode 14 and positiveelectrode 11 can be performed by using the liftoff method. Specifically,a photoresist is applied onto the surface on which the electrode will beformed, and thereafter, the photoresist is partially irradiated withultraviolet rays by using an UV exposure machine including a photomask.Thereafter, the photoresist is immersed in a developer to dissolve theexposed photoresist, and thereby, a resist film having a desired patternis formed. A metal film that should form the electrode is deposited onthe patterned resist film. Then, the resist film is dissolved by astripping solution and the metal film located on the resist film isremoved. In this manner, the metal film located in the region not havingthe resist film is left and the metal film (electrode) having aprescribed pattern is formed.

Examples of the method for patterning the electrode further include thefollowing method. Specifically, a metal film that should form theelectrode is formed on the electrode forming surface (e.g., exposedsurface of n-type semiconductor layer 15). Then, a photoresist isapplied onto the metal film, and thereafter, an exposing step and adeveloping step are performed to pattern the photoresist. Thereafter, byusing the aforementioned patterned photoresist (resist film) as a mask,the metal film is partially removed by dry etching or wet etching.Thereafter, the photoresist is dissolved by a stripping solution. Inthis manner as well, the electrode can be formed. The aforementionedliftoff method is suitably used because the process thereof is simplerthan that of the patterning method in which the resist pattern is formedon the metal film.

An arbitrary method such as a vacuum vapor deposition method, asputtering method and a chemical vapor deposition method can be used asa method for depositing the metal film forming negative electrode 14.However, from the perspective of eliminating an impurity in the metalfilm, it is preferable to use the vacuum vapor deposition method.Although various materials can be used for negative electrode 14, amaterial of negative electrode 14 can be selected from the knownmaterials. In order to enhance the contact property between n-typesemiconductor layer 15 and negative electrode 14, it is preferable toperform heat treatment at a temperature of not less than 300° C. and notmore than 1100° C. for a heating time of not less than 30 seconds andnot more than 3 minutes, after the metal film that should form negativeelectrode 14 is deposited. As to the temperature and the heating time ofheat treatment, heat treatment may be performed under optimum conditionsas appropriate, depending on a type of the metal forming negativeelectrode 14 and a film thickness of the metal film.

<Positive Electrode Layer>

Positive electrode 11 is formed on the p-type contact layer in p-typesemiconductor layer 12. Similarly to patterning of negative electrode14, it is preferable to use the liftoff method for patterning ofpositive electrode 11. Although various metal materials can be used forpositive electrode 11, a metal material of positive electrode 11 can beselected from the known materials. In addition, since it is preferablethat positive electrode 11 has translucency, thinner positive electrode11 is more preferable. Specifically, a thickness of positive electrode11 is not more than 10 nm, and more suitably not more than 5 nm.

Similarly to formation of negative electrode 14, examples of a methodfor depositing a metal film that should form positive electrode 11include the vacuum vapor deposition method, the sputtering method, thechemical vapor deposition method and the like. However, in order toeliminate an impurity in the metal film as much as possible, it ispreferable to use the vacuum vapor deposition method. In order toenhance the contact property with the p-type contact layer, it ispreferable to perform heat treatment at a temperature of not less than200° C. and not more than 800° C. for a time of not less than 30 secondsand not more than 3 minutes, after the metal film that should formpositive electrode 11 is deposited. As to the temperature and the timeof heat treatment, a suitable condition can be selected as appropriate,depending on a type of the metal forming positive electrode 11 and athickness of positive electrode 11.

The aforementioned semiconductor light emitting element is manufacturedby a manufacturing process shown in FIG. 5. Specifically, referring toFIG. 5, a substrate preparation step (S10) is first performed. In thisstep (S20), the substrate made of MN is prepared. It should be notedthat in this stage, periodic recessed and projecting structure 21 is notyet formed on the rear surface of the substrate. Next, a semiconductorlayer formation step (S20) is performed. In this step (S10), the stackedsemiconductor layer formed of p-type semiconductor layer 12, activelayer 13 and n-type semiconductor layer 15 is formed on the main surfaceof substrate 16. As described above, each of these p-type semiconductorlayer 12, active layer 13 and n-type semiconductor layer 15 can beformed by an arbitrary method such as the MOCVD method and the MOVPEmethod.

Next, an electrode formation step (S30) is performed. In this step, apart of p-type semiconductor layer 12, active layer 13 and n-typesemiconductor layer 15 is removed by etching, and thus, the exposedsurface of n-type semiconductor layer 15 is formed as shown in FIG. 1.By using the liftoff method, positive electrode 11 is formed on p-typesemiconductor layer 12 and negative electrode 14 is formed on theexposed surface of n-type semiconductor layer 15.

Thereafter, a recessed and projecting structure formation step (S40) isperformed. In this step (S40), a mask formation step (S41) is firstperformed. In this step (S41), the etching mask pattern is formed on therear surface of substrate 16 by using the lithography method asdescribed above. Next, an etching step (S42) is performed. In this step(S42), etching is performed on the rear surface of substrate 16 by usingthe aforementioned etching mask pattern as a mask. As a result, periodicrecessed and projecting structure 21 is formed. Next, a mask removalstep (S43) is performed. In this step (S43), the residue of the etchingmask is removed by an arbitrary method. In this manner, thesemiconductor light emitting element shown in FIG. 1 can be obtained.

Second Embodiment

A semiconductor light emitting element according to a second embodimentof the present invention basically has a structure similar to that ofthe semiconductor light emitting element shown in FIGS. 1 to 3. However,the semiconductor light emitting element according to the secondembodiment of the present invention is different from the semiconductorlight emitting element shown in FIGS. 1 to 3, in terms of aconfiguration of the rear surface of substrate 16. FIG. 6 conceptuallyshows a planar structure of the rear surface of substrate 16 of thesemiconductor light emitting element according to the second embodimentof the present invention. Referring to FIG. 6, in the semiconductorlight emitting element according to the second embodiment of the presentinvention, the rear surface of substrate 16 is used as one example ofthe light extraction surface, and periodic recessed and projectingstructure 21 is formed on the rear surface of substrate 16. A minuterecessed and projecting structure 22 is further formed on a surface ofperiodic recessed and projecting structure 21.

Specifically, the semiconductor light emitting element shown in FIG. 6is a semiconductor light emitting element comprising a semiconductorlayer including active layer 13 serving as a light emitting layer,wherein a surface of the semiconductor light emitting element includesthe rear surface of substrate 16 serving as the light extractionsurface. At least one of the light extraction surface and an interfacebetween two layers having different refractive indexes in thesemiconductor light emitting element is provided with periodic recessedand projecting structure 21 having period L1 that exceeds 0.5 times asgreat as a wavelength of light emitted from active layer 13, and minuterecessed and projecting structure 22 located on the surface of periodicrecessed and projecting structure 21 and having an average diameter(average value of a diameter D2) that is not more than 0.5 times asgreat as the wavelength of the light. The average diameter can bedetermined by measuring each diameter of minute recessed and projectingstructure 22 included in a square region with a side length being oneperiod of periodic recessed and projecting structure 21, and obtainingan average value of the diameters.

Here, description is given to the case in which the rear surface ofsubstrate 16 is used as one example of the light extraction surface andperiodic recessed and projecting structure 21 and the like are formed onthis rear surface 16A of substrate 16. However, the place where periodicrecessed and projecting structure 21 and minute recessed and projectingstructure 22 described above are formed is not limited to the lightextraction surface. Namely, the place where periodic recessed andprojecting structure 21 and minute recessed and projecting structure 22are formed may be an interface between layers having differentrefractive indexes in the semiconductor light emitting element.

For example, periodic recessed and projecting structure 21 and minuterecessed and projecting structure 22 described above may be formed on aninterface between rear surface 16A and a sealant (another member) whenanother member such as the sealant is formed on rear surface 16A ofsubstrate 16, an interface between layers having different refractiveindexes in the semiconductor layer or the like within the semiconductorlight emitting element, or other places.

When another member such as the sealant is formed on the rear surface ofsubstrate 16 as described above, a surface of this sealant (surface ofanother member) serves as the light extraction surface to the outside.In this case, periodic recessed and projecting structure 21 and minuterecessed and projecting structure 22 described above may also be formed,for example, on the sealant surface portion serving as the lightextraction surface, in addition to the interface having the largestdifference in refractive index (e.g., interface between rear surface 16Aand the sealant (another member)).

The semiconductor light emitting element according to the presentembodiment is basically similar to the semiconductor light emittingelement shown in FIGS. 1 to 3, in terms of the configurations of p-typesemiconductor layer 12, active layer 13, n-type semiconductor layer 15,positive electrode 11, and negative electrode 14. As described above,however, the semiconductor light emitting element according to thepresent embodiment is different from the semiconductor light emittingelement shown in FIGS. 1 to 3, in terms of the configuration ofsubstrate 16. Therefore, the configuration of substrate 16 will bedescribed below.

<Substrate>

The material and the properties of substrate 16 are basically similar tothose of substrate 16 in the semiconductor light emitting element shownin FIGS. 1 to 3, and sapphire, AlN, GaN and the like can, for example,be used. As described above, substrate 16 has periodic recessed andprojecting structure 21 on the light extraction surface (rear surface).Specifically, periodic recessed and projecting structure 21 includes aprojecting portion, and the projecting portion has a conical shape asshown in FIGS. 6 and 7. The projecting portion may have asemi-elliptical spherical shape as shown in FIG. 8.

Arrangement of periodic recessed and projecting structure 21 may beperiodic arrangement such as triangular lattice arrangement, squarelattice arrangement and hexagonal lattice arrangement, and is preferablytriangular lattice arrangement that achieves the maximum filling factor.Furthermore, periodic recessed and projecting structure 21 may haveperiod L1 that exceeds 0.5 times as great as the emission wavelength ofthe semiconductor light emitting element. In addition, it is preferablethat a height of periodic recessed and projecting structure 21 (heightH1 of the projecting portion) is within a range of not less than ⅓ timesand not more than 5 times as great as period L1.

The numerical range of period L1 of periodic recessed and projectingstructure 21 described above can be set to be, for example, not lessthan ⅔ times and not more than 1000 times, or not less than twice andnot more than 100 times as great as the aforementioned emissionwavelength. With such a configuration, the light extraction efficiencycan be enhanced more reliably, and the manufacturing cost can bereduced, and more uniform element shape and light output can beobtained. In addition, height H1 of periodic recessed and projectingstructure 21 is preferably not less than ½ times and not more than 3times, and more preferably not less than ¾ times and not more thantwice, as great as period L1. With such a configuration as well, thelight extraction efficiency can be enhanced more reliably, and themanufacturing cost can be reduced, and more uniform element shape andlight output can be obtained.

Furthermore, on the rear surface of substrate 16 having periodicrecessed and projecting structure 21 formed thereon, minute recessed andprojecting structure 22 smaller than periodic recessed and projectingstructure 21 is formed on the surface of periodic recessed andprojecting structure 21. Minute recessed and projecting structure 22includes a minute projecting portion. Minute recessed and projectingstructure 22 is arranged on a surface of the projecting portion ofperiodic recessed and projecting structure 21 and on a recessed portion(a flat portion located between the projecting portions) of periodicrecessed and projecting structure 21. It is preferable that the averagediameter of minute recessed and projecting structure 22 is not more thana half of the emission wavelength of the semiconductor light emittingelement, and a height of minute recessed and projecting structure 22 iswithin a range of not less than 0.1 times and not more than 10 times asgreat as the average diameter. The height of minute recessed andprojecting structure 22 is more preferably not less than 0.2 times andnot more than 5 times, and further preferably not less than 0.5 timesand not more than twice. It is preferable that the minute projectingportion of minute recessed and projecting structure 22 has a conicalshape or a semi-elliptical spherical shape.

The average diameter of minute recessed and projecting structure 22described above is more preferably not less than 1/30 times and not morethan ⅖ times, and further preferably not less than 1/10 times and notmore than 3/10 times, as great as the aforementioned emissionwavelength. With such a configuration, the light extraction efficiencycan be enhanced more reliably. In addition, an average height of minuterecessed and projecting structure 22 is preferably not less than 0.2times and not more than 5 times, and more preferably not less than 0.5times and not more than twice, as great as the average diameter. Withsuch a configuration as well, the light extraction efficiency can beenhanced more reliably.

The average height of minute recessed and projecting structure 22 can bedetermined by measuring each height of the minute recessed andprojecting structure included in a square region with a side lengthbeing one period of the periodic recessed and projecting structure, andobtaining an average value of the heights.

Next, a method for fabricating the recessed and projecting structurewill be described below. Periodic recessed and projecting structure 21and minute recessed and projecting structure 22 described above can befabricated by a process including firstly an etching mask fabricationstep (step (S410) in FIG. 9), secondly an etching step (step (S420) inFIG. 9), and thirdly an etching mask removal step (step (S430) in FIG.9). The etching mask fabrication step is a step of fabricating anetching mask pattern on the substrate, and the electron beam lithographymethod, the optical lithography method, the nanoimprint lithographymethod and the like can be applied. Alternatively, in order to enhancean etching selectivity in the etching step, a metal mask pattern may befabricated by forming a mask pattern (e.g., resist mask) having apattern by the aforementioned arbitrary method, and then, depositing ametal to cover the mask pattern, and then, removing a part of the metaltogether with the mask pattern by the liftoff method.

In formation of minute recessed and projecting structure 22, it ispreferable to form a metal mask by the liftoff method, and it is morepreferable that the metal mask is formed of a nickel film. A reason forthis is that a nickel particle etched in the etching step (S420) or areactant of nickel and the etching gas adheres to the rear surface ofsubstrate 16 again and acts as a nanosized etching mask, and thus,minute recessed and projecting structure 22 can be reliably formed.

By using the mask pattern as the etching mask, the rear surface ofsubstrate 16 is etched to form a desired pattern on the rear surface ofsubstrate 16. Dry etching such as inductively-coupled plasma (ICP)etching and reactive ion etching (RIE), wet etching using an acidsolution or an alkaline solution as an etchant, or the like can beapplied as a method for etching. In order to form a highly periodicpattern, it is preferable to apply dry etching.

In the etching step using dry etching, a resin material such as a resistor a metal can be used as the etching mask. Furthermore, a reactive gas,preferably a chlorine-based gas, a fluorine-based gas, a bromine-basedgas, or a gas obtained by mixing hydrogen, oxygen, argon or the likewith the etching gas can be used as the etching gas. In order to formminute recessed and projecting structure 22, it is preferable to use afluorine-based gas, particularly a carbon-containing fluorine-based gasas the gas for dry etching.

Alternatively, prior to the aforementioned mask fabrication step, therear surface of substrate 16 may be preliminarily roughened by dryetching or wet etching. In this case, periodic recessed and projectingstructure 21 is formed on this roughened surface by the aforementionedprocess, and thus, a combination of periodic recessed and projectingstructure 21 and minute recessed and projecting structure 22 can befabricated.

Alternatively, after periodic recessed and projecting structure 21 isformed, a fine particle of metal or ceramics may be arranged on the rearsurface (surface having periodic recessed and projecting structure 21formed thereon) of substrate 16 and dry etching may be performed byusing the fine particle as the etching mask. In this manner as well, thecombination of periodic recessed and projecting structure 21 and minuterecessed and projecting structure 22 can be fabricated. Examples of amethod for arranging the aforementioned fine particle include a methodfor applying a solvent having the fine particle dissolved therein ontothe rear surface of substrate 16 and drying the solvent, a method forforming a metal thin film on the rear surface of substrate 16 and thenheating the metal thin film and flocculating a metal of the metal thinfilm, and other methods. However, any of these methods may be used.

As described above, there are various fabrication methods for allowingperiodic recessed and projecting structure 21 and minute recessed andprojecting structure 22 to coexist. However, considering the simplicityof the process and the like, the method for etching the metal mask bythe carbon-containing fluorine-based gas is the most preferable.

After the etching step, a residue of the etching mask is removed as themask removal step. The method described in the first embodiment can beused as a method for removing the residue of the etching mask.

A sealing portion such as resin, glass and quartz may be formed on thecombination of periodic recessed and projecting structure 21 and minuterecessed and projecting structure 22. Furthermore, the combination ofperiodic recessed and projecting structure 21 and minute recessed andprojecting structure 22, or periodic recessed and projecting structure21 or minute recessed and projecting structure 22 may be formed on asurface of the sealing portion.

The aforementioned semiconductor light emitting element according to thepresent embodiment is manufactured by a manufacturing process shown inFIG. 9. Specifically, referring to FIG. 9, a substrate preparation step(S100) to an electrode formation step (S300) are performed. These steps(S100) to (S300) can be performed basically similarly to the steps (S10)to (S30) shown in FIG. 5.

Thereafter, a recessed and projecting structure formation step (S400) isperformed. In this step (S400), a mask formation step (S410) is firstperformed. In this step (S410), an etching mask pattern made of metal isformed on the rear surface of substrate 16 by using the lithographymethod and the like as described above. Next, an etching step (S420) isperformed. In this step (S420), by using the aforementioned etching maskpattern as a mask, etching is performed on the rear surface of substrate16 with the carbon-containing fluorine-based gas. As a result, periodicrecessed and projecting structure 21 and minute recessed and projectingstructure 22 are formed. Next, a mask removal step (S430) is performed.In this step (S430), a residue of the etching mask is removed by anarbitrary method. In this manner, the semiconductor light emittingelement shown in FIG. 6 can be obtained.

Periodic recessed and projecting structure 21 and minute recessed andprojecting structure 22 described above may be formed on an interfacebetween two layers having different refractive indexes in thesemiconductor light emitting element, not on the rear surface ofsubstrate 16. In this case as well, a ratio of reflection and totalreflection, from this interface, of the light emitted from the lightemitting layer can be reduced, and as a result, the light extractionefficiency of the semiconductor light emitting element can be enhanced.

Hereinafter, the characteristic configuration of the present inventionwill be described, some of which may duplicate those described in theabove embodiments.

Specifically, one aspect of the present invention is characterized inthat periodic recessed and projecting structure 21 having a period thatexceeds 0.5 times as great as an emission wavelength and minute recessedand projecting structure 22 having an average diameter that is not morethan a half of the emission wavelength are formed together on the samesurface of the semiconductor light emitting element (such as rearsurface 16A of substrate 16 which is one example of the light extractionsurface, or the surface of another member when another member such asthe sealant is arranged on rear surface 16A of substrate 16) or on thesame interface between layers having different refractive indexes in thesemiconductor light emitting element (such as, for example, an interfacebetween rear surface 16A and a resin when the resin or the like isarranged on rear surface 16A of substrate 16, or the interface betweenthe layers having different refractive indexes in the semiconductorlayer or the like within the semiconductor light emitting element).

In this case, minute recessed and projecting structure 22 smaller thanthe periodic recessed and projecting structure is formed on the flatsurface portion located between periodic recessed and projectingstructures 21 and on the surface of periodic recessed and projectingstructure 21. Therefore, as compared with the case in which periodicrecessed and projecting structure 21 is present alone, a difference inrefractive index at the surface or the interface can be furthermitigated, and reflection and total reflection can be suppressed.

In the case of periodic recessed and projecting structure 21 alone, itis normally necessary to form small-scale periodic recessed andprojecting structure 21 that is approximately equal to or smaller thanthe wavelength, in order to enhance the light extraction efficiency.However, in the structure according to the present embodiment, even ifperiodic recessed and projecting structure 21 has a size larger than thewavelength, the light extraction efficiency can be sufficiently enhancedby combining periodic recessed and projecting structure 21 with minuterecessed and projecting structure 22. Namely, even if the emissionwavelength is short, the cost required to fabricate the light extractionstructure is reduced, and fabrication of the uniform structure becomeseasy because a process window is widened.

Furthermore, it is preferable that an arrangement pattern of periodicrecessed and projecting structure 21 is a triangular lattice-likepattern.

Furthermore, as for the shape of periodic recessed and projectingstructure 21, it is preferable that a cross-sectional area of a highrefractive index medium decreases from the bottom toward the vertexdirection (light extraction direction).

Furthermore, it is preferable that periodic recessed and projectingstructure 21 has a projecting shape and the projecting shape is conicalor semi-elliptical spherical.

The present invention is not limited to the following, while one aspectof the present invention has the following characteristics.

One aspect of the present invention is characterized by including astacked semiconductor structure having an n-type group-III nitridesemiconductor layer (n-type semiconductor layer 15), a group-III nitridesemiconductor light emitting layer (active layer 13) and a p-typegroup-III nitride semiconductor layer (p-type semiconductor layer 12).

One aspect of the present invention is characterized by including atransparent substrate (substrate 16) having a flip chip structure andhaving transparency to the emission wavelength on the light extractionsurface side as seen from the group-III nitride semiconductor lightemitting layer.

One aspect of the present invention is characterized in that thetransparent substrate is an aluminum nitride (AlN) substrate or asapphire substrate.

One aspect of the present invention is characterized in that theemission wavelength is equal to or shorter than 450 nm, or equal to orshorter than 350 nm.

One aspect of the present invention is characterized in that a height ofperiodic recessed and projecting structure 21 is within a range of ⅓times to 5 times as great as the period, and an average height of minuterecessed and projecting structure 22 is within a range of 1/10 times to5 times as great as the average diameter.

Another aspect of the present invention is characterized by including astacked semiconductor structure having an AlN substrate (substrate 16),an n-type group-III nitride semiconductor layer (n-type semiconductorlayer 15), a group-III nitride semiconductor light emitting layer(active layer 13), and a p-type group-III nitride semiconductor layer(p-type semiconductor layer 12), wherein an emission wavelength is equalto or shorter than 350 nm, and periodic recessed and projectingstructure 21 having a period that is within a range of ⅓ times to 5times as great as the emission wavelength/(a difference between arefractive index of the AlN substrate and a refractive index of anexternal medium) is formed on a surface of the AlN substrate.

Furthermore, it is preferable that an arrangement pattern of periodicrecessed and projecting structure 21 is a triangular lattice-likepattern.

Furthermore, it is preferable that periodic recessed and projectingstructure 21 has a projecting shape and the projecting shape is conicalor semi-elliptical spherical. Furthermore, it is preferable that aheight of periodic recessed and projecting structure 21 is within arange of ⅓ times to 5 times as great as the period.

Still another aspect of the present invention is a method formanufacturing the aforementioned semiconductor light emitting element,characterized by including the steps of: periodically processing anorganic thin film; forming a metal mask by using the organic film(organic thin film); and forming periodic recessed and projectingstructure 21 by a dry etching method by using the mask.

Furthermore, still another aspect of the present invention ischaracterized by including the step of simultaneously forming periodicrecessed and projecting structure 21 and minute recessed and projectingstructure 22 by the dry etching method using the metal mask and afluorine-based gas.

In this case, by the dry etching method using the metal mask and thefluorine-based gas, the periodic recessed and projecting structure canbe artificially fabricated with uniformity and a high degree ofaccuracy. In addition, by acid treatment for stripping the metal maskafter dry etching, minute recessed and projecting structure 22sufficiently smaller than the wavelength is spontaneously formed on theflat surface portion located between the periodic recessed andprojecting structures and on the surface of periodic recessed andprojecting structure 21. Therefore, periodic recessed and projectingstructure 21 and minute recessed and projecting structure 22 can besimultaneously formed in one process. Therefore, periodic recessed andprojecting structure 21, which is as great as or greater than thewavelength and whose shape change affects the properties greatly, can befabricated with uniformity and a high degree of accuracy, while minuterecessed and projecting structure 22, which is sufficiently smaller thanthe wavelength and whose shape change does not affect the propertiesgreatly, can be formed spontaneously and densely. In addition, periodicrecessed and projecting structure 21 and minute recessed and projectingstructure 22 are characterized by being made of the same material. As aresult, the uniformity of the processed shape and the reproducibility ofthe process can be enhanced, and the light extraction efficiency and theuniformity thereof can be enhanced, and further, the manufacturing costcan be lowered.

According to the present invention, by combining periodic recessed andprojecting structure 21 and minute recessed and projecting structure 22having different scales, reflection and total reflection from thesubstrate surface (light extraction surface) or the interface areeffectively suppressed. In addition, since the process window iswidened, the semiconductor light emitting element that achieves highlight extraction efficiency and uniform light output can be fabricatedwith high reproducibility and high productivity, even if the emissionwavelength is short. Furthermore, according to the present invention,due to the manufacturing method in which periodic recessed andprojecting structure 21 and minute recessed and projecting structure 22are simultaneously fabricated, the uniformity of the processed shape andthe reproducibility of the process can be enhanced, and the lightextraction efficiency and the uniformity thereof can be enhanced, andfurther, the manufacturing cost can be lowered.

EXAMPLE 1

Based on the structure of the semiconductor light emitting elementaccording to the aforementioned embodiment of the present invention, asemiconductor light emitting element according to Example 1 wasfabricated as shown in FIGS. 10 and 11. Specifically, n-typesemiconductor layer 15, active layer 13 (light emitting layer) andp-type semiconductor layer 12 were sequentially grown on substrate 16made of single-crystal MN by the MOCVD method to form a light emittingelement substrate, and positive electrode 11 and negative electrode 14were arranged at prescribed positions of the light emitting elementsubstrate. An epitaxial layer including the light emitting layer of thesemiconductor light emitting element was formed of an AlGaN-basedsemiconductor similar to that in the aforementioned embodiment, and anemission wavelength of the element was 265 nm.

An electron beam resist was applied onto a substrate surface (lightextraction surface) of the fabricated semiconductor light emittingelement substrate opposite to the epitaxial layer, and alignment wasperformed to cover a light emitting portion of the semiconductor lightemitting element, and electron beam lithography was performed tofabricate an etching mask pattern. The light emitting portion was acircular region having a diameter of 100 μm, and a lithography regionwas 900 μm×900 μm, with a center of the light emitting portion being acenter of lithography. A lithography pattern was set to have a diameterof 220 nm, a pattern period of 300 nm, and triangular lattice patternarrangement. Next, nickel was deposited in a thickness of 100 nm to 500nm on the etching mask pattern by the vacuum vapor deposition method. Asdescribed in the aforementioned embodiment, a reason why nickel wasdeposited was to enhance the etching selectivity of substrate 16 and theetching mask pattern. After nickel was deposited, the semiconductorlight emitting element substrate was immersed in a stripping solutionfor the electron beam resist, and the resist and nickel located on thisresist were removed (liftoff method). In this manner, a mask patternmade of nickel was formed on the rear surface of substrate 16.

Then, the aforementioned semiconductor light emitting element substratewas introduced into an ICP etching apparatus, and etching treatment wasperformed for 10 to 30 minutes by using a trifluoromethane (CHF₃) gas.Thereafter, in order to remove the mask pattern made of nickel, thesemiconductor light emitting element substrate was immersed for 15minutes in hydrochloric acid having a temperature of 20° C. to 30° C. Atthis time, in order to prevent the electrode metal of the semiconductorlight emitting element substrate from becoming eroded by hydrochloricacid, a photoresist was preliminarily applied onto the surface of thesemiconductor light emitting element substrate having the electrodesformed thereon, and was cured and used as a protective film. Afterimmersion in hydrochloric acid, the semiconductor light emitting elementsubstrate was rinsed with ultrapure water, and the photoresist servingas a protective film was dissolved by the stripping solution.

In this manner, the ultraviolet-light-emitting semiconductor lightemitting element of Example 1 including substrate 16 having a conicalstructure having a conical bottom diameter of 250 nm, period L1 of 300nm and height H1 of 250 nm was fabricated. SEM photographs of thefabricated recessed and projecting structure are shown in FIGS. 12 and13.

As a comparative example with respect to Example 1, anultraviolet-light-emitting semiconductor light emitting element beforeforming the recessed and projecting structure on substrate 16 wasprepared (Comparative Example 1). Then, the light output of the samplesof these Example and Comparative Example 1 was measured. The result isshown in FIG. 14.

Referring to FIG. 14, the horizontal axis represents the light outputratio in the Example with respect to Comparative Example 1, and thevertical axis represents the number of samples. Assuming that the lightoutput in Comparative Example 1 was 1.00, an average value of the lightoutput ratio in the Example was 1.31. FIG. 14 is a histogram showing thelight output ratio of the ultraviolet-light-emitting semiconductor lightemitting element which is the sample of Example 1. In addition, astandard deviation of the light output ratio in Example 1 was 0.031,which corresponded to 2.3% of the average value of the light outputratio. Namely, the sample of Example 1 was proved to be a semiconductorlight emitting element having extremely small variations in lightemission output.

EXAMPLE 2

Based on the structure of the semiconductor light emitting elementaccording to the aforementioned embodiment of the present invention, asemiconductor light emitting element according to Example 2 wasfabricated. The configuration of the semiconductor light emittingelement according to Example 2 was basically similar to that of thesemiconductor light emitting element according to Example 1.Specifically, n-type semiconductor layer 15, active layer 13 (lightemitting layer) and p-type semiconductor layer 12 were sequentiallygrown on substrate 16 made of single-crystal MN by the MOCVD method toform a light emitting element substrate, and positive electrode 11 andnegative electrode 14 were arranged at prescribed positions of the lightemitting element substrate. An epitaxial layer including the lightemitting layer of the semiconductor light emitting element was formed ofan AlGaN-based semiconductor similar to that in the aforementionedembodiment, and an emission wavelength of the element was 265 nm.

An electron beam resist was applied onto a substrate surface (lightextraction surface) of the fabricated semiconductor light emittingelement wafer opposite to the light emitting element layer, andalignment was performed to cover a light emitting portion of thesemiconductor light emitting element, and electron beam lithography wasperformed to fabricate an etching mask pattern. The light emittingportion was a circular region having a diameter of 100 μm, and alithography region was 900 μm×900 μm, with a center of the lightemitting portion being a center of lithography. A lithography patternwas set to have a diameter of 300 nm, a pattern period of 600 nm, andequilateral-triangular lattice pattern arrangement. Next, nickel wasdeposited in a thickness of 100 nm to 500 nm on the mask pattern by thevacuum vapor deposition method. A reason why nickel was deposited wassimilar to the reason described in Example 1. After nickel wasdeposited, the semiconductor light emitting element substrate wasimmersed in a stripping solution for the electron beam resist, and theresist and nickel located on this resist were removed (liftoff method).In this manner, a mask pattern made of nickel was formed on the rearsurface of substrate 16.

Then, the aforementioned semiconductor light emitting element substratewas introduced into an ICP etching apparatus, and etching treatment wasperformed for 30 to 80 minutes by using a trifluoromethane (CHF₃) gas.By adjusting the nickel film thickness and the etching time, thepresence or absence of development of a minute recessed and projectingstructure and the shape thereof were controlled. Finally, in order toremove the mask pattern made of nickel, the semiconductor light emittingelement substrate was immersed for 15 minutes in hydrochloric acidheated to a temperature of 60° C. to 90° C. At this time, in order toprevent the electrode metal of the semiconductor light emitting elementsubstrate from becoming eroded by hydrochloric acid, a photoresist waspreliminarily applied onto the surface of the semiconductor lightemitting element substrate having the electrodes formed thereon, and wascured and used as a protective film. After immersion in hydrochloricacid, the semiconductor light emitting element substrate was rinsed withultrapure water, and the photoresist used as a protective film wasdissolved by the stripping solution.

In this manner, the ultraviolet-light-emitting semiconductor lightemitting element of Example 2 including the substrate having theperiodic recessed and projecting structure having a conical bottomdiameter of 600 nm, a period of 600 nm and a height of 550 nm and theminute recessed and projecting structure having an average diameter of52 nm and an average height of 52 nm was fabricated. SEM photographs ofthe fabricated recessed and projecting structure are shown in FIGS. 15to 17.

As a comparative example with respect to Example 2, anultraviolet-light-emitting semiconductor light emitting element beforeforming the recessed and projecting structure on the substrate wasprepared (Comparative Example 2). Then, the light output of thesesamples of Example 2 and Comparative Example 2 was measured. The resultis shown in FIG. 18.

Referring to FIG. 18, the horizontal axis represents the light outputratio in Example 2 with respect to Comparative Example 2, and thevertical axis represents the number of samples. Assuming that the lightoutput in Comparative Example 2 was 1.00, an average value of the lightoutput ratio in Example 2 was 1.70. Assuming that anultraviolet-light-emitting semiconductor light emitting element havingonly the minute recessed and projecting structure was ComparativeExample 3, an average value of the light output ratio in ComparativeExample 3 with respect to Comparative Example 2 was 1.25. As a result,the superiority of the structure having both the periodic recessed andprojecting structure and the minute recessed and projecting structurefabricated in Example 2 could be proved. FIG. 18 is a histogram showingthe light output ratio of the ultraviolet-light-emitting semiconductorlight emitting element which is the sample of Example 2. In addition, astandard deviation of the light output ratio in Example 2 was 0.029,which corresponded to 1.7% of the average value of the light outputratio in Example 2. Namely, the sample of Example 2 was also proved tobe a semiconductor light emitting element having extremely smallvariations in light emission output.

EXAMPLE 3

In order to check the effect of periodic recessed and projectingstructure 21 formed in the semiconductor light emitting elementaccording to the present invention, simulation calculation describedbelow was performed. Specifically, calculation was performed of thelight extraction efficiency at which the light (wavelength: 265 nm)emitted from the AlGaN layer serving as the light emitting layer wasextracted to the outside (air) through the AlN substrate and theperiodic recessed and projecting structure (conical two-dimensionalperiodic arrangement (triangular lattice) made of AlN) processed on thesurface of the AlN substrate. Calculation was also performed of thelight extraction efficiency when there is no periodic recessed andprojecting structure in the similar system.

The finite-difference time-domain method (FDTD method) was used forcalculation and a dipole point light source was set as an initial lightsource. Oscillation directions and positions of the dipole were changed,and calculation and averaging (pseudo randomization) were performed toartificially reproduce a non-coherent light source. A refractive indexof the AlGaN portion was assumed as 2.43, a refractive index of the AlNportion was assumed as 2.29, and a refractive index of the air portionwas assumed as 1.0. The side (rear surface side) opposite to the lightextraction surface as seen from the light emitting layer was anabsorbing boundary because the light is normally absorbed by the p-GaNlayer. The results are shown in FIGS. 19 and 20.

FIGS. 19 and 20 show numerical values (light output ratio) obtained bycalculating the light extraction efficiency at which the light(wavelength: 265 nm) emitted from the AlGaN layer serving as the lightemitting layer is extracted to the outside (air) through the AlNsubstrate and the periodic recessed and projecting structure (conicaltwo-dimensional periodic arrangement (triangular lattice) made of AlN)processed on the surface of the AlN substrate, and standardizing thislight extraction efficiency with the result when there is no periodicrecessed and projecting structure (flat surface). A width of the bottomof the projecting portion (conical portion) of the periodic recessed andprojecting structure was matched with the period. In FIG. 19, thehorizontal axis represents the period (unit: nm) of the periodicrecessed and projecting structure, and the vertical axis represents thelight output ratio. In FIG. 19, data is shown for each of differentaspect ratios. In FIG. 20, the horizontal axis represents the aspectratio (a ratio of a height of the projecting portion to the width of thebottom of the projecting portion (conical portion) of the periodicrecessed and projecting structure), and the vertical axis represents thelight output ratio. In FIG. 20, data is shown for each period (a) of theperiodic recessed and projecting structure.

Referring to FIG. 19, in the period range of 200 nm to 450 nm, the lightoutput ratio is the highest when the aspect ratio is 1.0. Referring toFIG. 20, the light output ratio is the highest when the aspect ratio is1.0.

FIGS. 19 and 20 show the results of calculation in two dimensions.However, it has been confirmed that the results showing a similartendency to that of calculation in three dimensions are obtained.

EXAMPLE 4

In order to check the effect of periodic recessed and projectingstructure 21 formed in the semiconductor light emitting elementaccording to the present invention, simulation calculation describedbelow was performed. Specifically, calculation was performed of thelight extraction efficiency at which the light (wavelength: 265 nm)emitted from the AlGaN layer serving as the light emitting layer wasextracted to the outside (sealant layer) through the AlN substrate andthe periodic recessed and projecting structure (conical two-dimensionalperiodic arrangement (triangular lattice) made of AlN) processed on thesurface of the AlN substrate. Calculation was also performed of thelight extraction efficiency when there is no periodic recessed andprojecting structure in the similar system. The calculation method wassimilar to that in Example 3. A refractive index of the AlGaN portionwas assumed as 2.43, a refractive index of the AlN portion was assumedas 2.29, and a refractive index of the sealant portion was assumed as1.45. SiO₂, resin and the like were assumed as the sealant portion. Theother conditions were similar to those in Example 3.

FIGS. 21 and 22 show numerical values (light output ratio) obtained bycalculating the light extraction efficiency at which the light(wavelength: 265 nm) emitted from the AlGaN layer is extracted to theoutside (sealant layer) through the AlN substrate and the periodicrecessed and projecting structure (conical two-dimensional periodicarrangement (triangular lattice) made of AlN) processed on the surfaceof the AlN substrate, and standardizing this light extraction efficiencywith the calculation result of the light extraction efficiency at whichthe light is extracted from the flat surface to the outside (air layer)when there is no periodic recessed and projecting structure on thesurface of the AlN substrate.

In FIG. 21, the horizontal axis represents the period (unit: nm) of theperiodic recessed and projecting structure, and the vertical axisrepresents the light output ratio. In FIG. 21, data is shown for each ofdifferent aspect ratios. In FIG. 22, the horizontal axis represents theaspect ratio, and the vertical axis represents the light output ratio.In FIG. 22, data is shown for each period (a) of the periodic recessedand projecting structure.

Based on the results of Examples 3 and 4, it turns out that, even in thecase of the same substrate, wavelength and periodic recessed andprojecting structure, the optimum light extraction structure variesdepending on the refractive index of the external medium such as thesealing member. However, from the perspective of the production process,it is preferable to extract the light to either the air or the sealantlayer from the AlN substrate and the periodic recessed and projectingstructure processed on the surface of the AlN substrate, and based onthese results, the effect of periodic recessed and projecting structure21 can be confirmed.

EXAMPLE 5

Based on the structure of the semiconductor light emitting elementaccording to the aforementioned embodiment of the present invention, asemiconductor light emitting element according to Example 5 wasfabricated. The configuration of the semiconductor light emittingelement according to Example 5 was basically similar to that of thesemiconductor light emitting element according to Example 1. Anepitaxial layer including the light emitting layer of the semiconductorlight emitting element was formed of an AlGaN-based semiconductorsimilar to that in the aforementioned embodiment, and an emissionwavelength of the element was 265 nm.

An electron beam resist was applied onto a substrate surface (lightextraction surface) of the fabricated semiconductor light emittingelement wafer opposite to the light emitting element layer, andalignment was performed to cover a light emitting portion of thesemiconductor light emitting element, and electron beam lithography wasperformed to fabricate an etching mask pattern. The light emittingportion was a circular region having a diameter of 100 μm, and alithography region was 900 μm×900 μm, with a center of the lightemitting portion being a center of lithography. A lithography patternwas set to have a diameter of 180 nm, a pattern period of 300 nm, andequilateral-triangular lattice pattern arrangement. Next, nickel wasdeposited in a thickness of 100 nm to 500 nm on the mask pattern by thevacuum vapor deposition method. A reason why nickel was deposited wassimilar to the reason described in Example 1. After nickel wasdeposited, the semiconductor light emitting element substrate wasimmersed in a stripping solution for the electron beam resist, and theresist and nickel located on this resist were removed (liftoff method).In this manner, a mask pattern made of nickel was formed on the rearsurface of substrate 16.

Then, similarly to Example 2, the aforementioned semiconductor lightemitting element substrate was introduced into an ICP etching apparatus,and etching treatment was performed for 10 to 80 minutes by using atrifluoromethane (CHF₃) gas. In the structure like Example 5 having apattern size smaller than that of Example 2, the etching treatment timewas relatively shorter. Finally, in order to remove the mask patternmade of nickel, the semiconductor light emitting element substrate wasimmersed for 15 minutes in hydrochloric acid heated to a temperature of60° C. to 90° C. By adjusting the temperature of hydrochloric acid, thepresence or absence of development of the minute recessed and projectingstructure and the shape thereof could be controlled. Similarly toExample 2, in order to prevent the electrode metal of the semiconductorlight emitting element substrate from becoming eroded by hydrochloricacid, a photoresist was preliminarily applied onto the surface of thesemiconductor light emitting element substrate having the electrodesformed thereon, and was cured and used as a protective film. Afterimmersion in hydrochloric acid, the semiconductor light emitting elementsubstrate was rinsed with ultrapure water, and the photoresist used as aprotective film was dissolved by the stripping solution.

In this manner, the ultraviolet-light-emitting semiconductor lightemitting element of Example 5 including the substrate having theperiodic recessed and projecting structure having a conical bottomdiameter of 300 nm, a period of 300 nm and an aspect ratio of 1 and theminute recessed and projecting structure having an average diameter of33 nm and an average height of 33 nm was fabricated.

As a comparative example with respect to Example 5, anultraviolet-light-emitting semiconductor light emitting element beforeforming the recessed and projecting structure on the substrate wasprepared (Comparative Example 4). Then, the light output of thesesamples of Example 5 and Comparative Example 4 was measured. The resultis shown in FIG. 23.

Referring to FIG. 23, the horizontal axis represents the light outputratio in Example 5 with respect to Comparative Example 4, and thevertical axis represents the number of samples. Assuming that the lightoutput in Comparative Example 4 was 1.00, an average value of the lightoutput ratio in Example 5 was 1.96. As can be seen from FIG. 23, thehigh light output ratio was obtained in Example 5, and the superiorityof the structure having both the periodic recessed and projectingstructure and the minute recessed and projecting structure could beproved. In addition, a standard deviation of the light output ratio inExample 5 was 0.07, which corresponded to 3.6% of the average value ofthe light output ratio. As described above, the sample of Example 5 wasalso proved to be a semiconductor light emitting element havingrelatively small variations in light output.

EXAMPLE 6

Based on the structure of the semiconductor light emitting elementaccording to the aforementioned embodiment of the present invention, asemiconductor light emitting element according to Example 6 wasfabricated. The configuration of the semiconductor light emittingelement according to Example 6 was basically similar to that of thesemiconductor light emitting element according to Example 1. A materialof an epitaxial layer including the light emitting layer of thesemiconductor light emitting element and an emission wavelength of theelement were similar to those in Example 5 described above.

Similarly to Example 5, by electron beam lithography, an etching maskpattern was fabricated on a substrate surface (light extraction surface)of the fabricated semiconductor light emitting element wafer opposite tothe light emitting element layer. The light emitting portion was acircular region having a diameter of 100 μm, and a lithography regionwas 900 μm×900 μm, with a center of the light emitting portion being acenter of lithography. A lithography pattern was set to have a diameterof 200 nm, a pattern period of 400 nm, and equilateral-triangularlattice pattern arrangement. Next, similarly to Example 5, nickel wasdeposited in a thickness of 100 nm to 500 nm on the mask pattern by thevacuum vapor deposition method. After nickel was deposited, thesemiconductor light emitting element substrate was immersed in astripping solution for the electron beam resist, and the resist andnickel located on this resist were removed (liftoff method). In thismanner, a mask pattern made of nickel was formed on the rear surface ofsubstrate 16.

Then, similarly to Example 2, the aforementioned semiconductor lightemitting element substrate was introduced into an ICP etching apparatus,and etching treatment was performed for 10 to 80 minutes by using atrifluoromethane (CHF₃) gas. Finally, in order to remove the maskpattern made of nickel, the semiconductor light emitting elementsubstrate was immersed for 15 minutes in hydrochloric acid heated to atemperature of 60° C. to 90° C. Similarly to Example 2, in order toprevent the electrode metal of the semiconductor light emitting elementsubstrate from becoming eroded by hydrochloric acid, a photoresist waspreliminarily applied onto the surface of the semiconductor lightemitting element substrate having the electrodes formed thereon, and wascured and used as a protective film. After immersion in hydrochloricacid, the semiconductor light emitting element substrate was rinsed withultrapure water, and the photoresist used as a protective film wasdissolved by the stripping solution.

In this manner, the ultraviolet-light-emitting semiconductor lightemitting element of Example 6 including the substrate having theperiodic recessed and projecting structure having a conical bottomdiameter of 400 nm, a period of 400 nm and an aspect ratio of 1 and theminute recessed and projecting structure having an average diameter of33 nm and an average height of 33 nm was fabricated.

As a comparative example with respect to Example 6, anultraviolet-light-emitting semiconductor light emitting element beforeforming the recessed and projecting structure on the substrate wasprepared (Comparative Example 5). Then, the light output of thesesamples of Example 6 and Comparative Example 5 was measured. The resultis shown in FIG. 24.

Referring to FIG. 24, the horizontal axis represents the light outputratio in Example 6 with respect to Comparative Example 5, and thevertical axis represents the number of samples. Assuming that the lightoutput in Comparative Example 5 was 1.00, an average value of the lightoutput ratio in Example 6 was 1.79. As can be seen from FIG. 24, thehigh light output ratio was obtained in Example 6 as well similarly toExample 5, and the superiority of the structure having both the periodicrecessed and projecting structure and the minute recessed and projectingstructure could be proved.

In addition, as can be seen from FIG. 24, similarly to the sample ofExample 5, the sample of Example 6 was also proved to be a semiconductorlight emitting element having relatively small variations in lightoutput.

EXAMPLE 7

Based on the structure of the semiconductor light emitting elementaccording to the aforementioned embodiment of the present invention, asemiconductor light emitting element according to Example 7 wasfabricated. The configuration of the semiconductor light emittingelement according to Example 7 was basically similar to that of thesemiconductor light emitting element according to Example 1. A materialof an epitaxial layer including the light emitting layer of thesemiconductor light emitting element and an emission wavelength of theelement were similar to those in Example 5 described above.

Similarly to Example 5, by electron beam lithography, an etching maskpattern was fabricated on a substrate surface (light extraction surface)of the fabricated semiconductor light emitting element wafer opposite tothe light emitting element layer. The light emitting portion was acircular region having a diameter of 100 μm, and a lithography regionwas 900 μm×900 μm, with a center of the light emitting portion being acenter of lithography. A lithography pattern was set to have a diameterof 400 nm, a pattern period of 1000 nm, and equilateral-triangularlattice pattern arrangement. Next, similarly to Example 5, nickel wasdeposited in a thickness of 100 nm to 500 nm on the mask pattern by thevacuum vapor deposition method. After nickel was deposited, thesemiconductor light emitting element substrate was immersed in astripping solution for the electron beam resist, and the resist andnickel located on this resist were removed (liftoff method). In thismanner, a mask pattern made of nickel was formed on the rear surface ofsubstrate 16.

Then, similarly to Example 2, the aforementioned semiconductor lightemitting element substrate was introduced into an ICP etching apparatus,and etching treatment was performed for 10 to 80 minutes by using atrifluoromethane (CHF₃) gas. In the structure like Example 5 having arelatively small pattern size, the aforementioned etching treatment timewas shorter. Conversely, in the structure like Example 7 having arelatively large pattern size, the aforementioned etching treatment timewas longer.

Finally, in order to remove the mask pattern made of nickel, thesemiconductor light emitting element substrate was immersed for 15minutes in hydrochloric acid heated to a temperature of 60° C. to 90° C.Similarly to Example 2, in order to prevent the electrode metal of thesemiconductor light emitting element substrate from becoming eroded byhydrochloric acid, a photoresist was preliminarily applied onto thesurface of the semiconductor light emitting element substrate having theelectrodes formed thereon, and was cured and used as a protective film.After immersion in hydrochloric acid, the semiconductor light emittingelement substrate was rinsed with ultrapure water, and the photoresistused as a protective film was dissolved by the stripping solution.

In this manner, the ultraviolet-light-emitting semiconductor lightemitting element of Example 7 including the substrate having theperiodic recessed and projecting structure having a conical bottomdiameter of 1000 nm, a period of 1000 nm and an aspect ratio of 1 andthe minute recessed and projecting structure having an average diameterof 33 nm and an average height of 33 nm was fabricated.

As a comparative example with respect to Example 7, anultraviolet-light-emitting semiconductor light emitting element beforeforming the recessed and projecting structure on the substrate wasprepared (Comparative Example 6). Then, the light output of thesesamples of Example 7 and Comparative Example 6 was measured. The resultis shown in FIG. 25.

Referring to FIG. 25, the horizontal axis represents the light outputratio in Example 7 with respect to Comparative Example 6, and thevertical axis represents the number of samples. Assuming that the lightoutput in Comparative Example 6 was 1.00, an average value of the lightoutput ratio in Example 7 was 1.69. As can be seen from FIG. 25, thehigh light output ratio was obtained in Example 7 as well similarly toExample 5, and the superiority of the structure having both the periodicrecessed and projecting structure and the minute recessed and projectingstructure could be proved. In addition, as can be seen from FIG. 25,similarly to the sample of Example 5, the sample of Example 7 was alsoproved to be a semiconductor light emitting element having relativelysmall variations in light output.

EXAMPLE 8

As a semiconductor light emitting element according to Example 8,fabricated was a semiconductor light emitting element in which theperiod of the periodic recessed and projecting structure (patternperiod: 300 nm) fabricated on the light extraction surface of theaforementioned semiconductor light emitting element wafer according toExample 1 was changed to 600 nm. In addition, in order to fix the aspectratio to 1, the diameter and the height were set to correspond to theaforementioned pattern period. The aforementioned semiconductor lightemitting element according to Example 8 was similar to the semiconductorlight emitting element according to Example 1, except for the patternperiod, the diameter and the height described above. The fabricationconditions were also similar to those in Example 1, except that thetreatment time of etching treatment was 30 to 80 minutes.

In this manner, the ultraviolet-light-emitting semiconductor lightemitting element including the substrate having the periodic recessedand projecting structure having a conical bottom diameter of 600 nm, aperiod of 600 nm and a height of 600 nm was fabricated as thesemiconductor light emitting element according to Example 8.

As a comparative example with respect to Example 8, anultraviolet-light-emitting semiconductor light emitting element beforeforming the recessed and projecting structure on the substrate wasprepared (Comparative Example 7). Then, the light output of thesesamples of Example 8 and Comparative Example 7 was measured. The resultis shown in FIG. 26.

Referring to FIG. 26, the horizontal axis represents the light outputratio in Example 8 with respect to Comparative Example 7, and thevertical axis represents the number of samples. Assuming that the lightoutput in Comparative Example 7 was 1.00, an average value of the lightoutput ratio in Example 8 was 1.446. As compared with Examples 2 and 8having the same periodic recessed and projecting structure (period: 600nm) as well as Comparative Example 7 not having the periodic recessedand projecting structure, the light output ratio decreases in the orderof Example 2>Example 8>Comparative Example 7, and the superiority of thestructure having both the periodic recessed and projecting structure andthe minute recessed and projecting structure could be proved.

EXAMPLE 9

As a semiconductor light emitting element according to Example 9,fabricated was a semiconductor light emitting element in which theperiod of the periodic recessed and projecting structure (patternperiod: 300 nm) fabricated on the light extraction surface of theaforementioned semiconductor light emitting element wafer according toExample 1 was changed to 1000 nm. In addition, in order to fix theaspect ratio to 1, the diameter and the height were set to correspond tothe aforementioned pattern period. The aforementioned semiconductorlight emitting element according to Example 9 was similar to thesemiconductor light emitting element according to Example 1, except forthe pattern period, the diameter and the height described above. Thefabrication conditions were also similar to those in Example 1, exceptthat the treatment time of etching treatment was 30 to 80 minutes.

In this manner, the ultraviolet-light-emitting semiconductor lightemitting element including the substrate having the periodic recessedand projecting structure having a conical bottom diameter of 1000 nm, aperiod of 1000 nm and a height of 1000 nm was fabricated as thesemiconductor light emitting element according to Example 9.

As a comparative example with respect to Example 9, anultraviolet-light-emitting semiconductor light emitting element beforeforming the recessed and projecting structure on the substrate wasprepared (Comparative Example 8). Then, the light output of thesesamples of Example 9 and Comparative Example 8 was measured. The resultis shown in FIG. 26.

Referring to FIG. 26, the horizontal axis represents the light outputratio in Example 9 with respect to Comparative Example 8, and thevertical axis represents the number of samples. Assuming that the lightoutput in Comparative Example 8 was 1.00, an average value of the lightoutput ratio in Example 9 was 1.26. As compared with Examples 7 and 9having the same periodic recessed and projecting structure (period: 1000nm) as well as Comparative Example 8 not having the periodic recessedand projecting structure, the light output ratio decreases in the orderof Example 7>Example 9>Comparative Example 8, and the superiority of thestructure having both the periodic recessed and projecting structure andthe minute recessed and projecting structure could be proved.

As shown in FIG. 27, the light output ratio obtained for Examples 1, 8and 9 described above is well matched with the calculation resultdescribed in Example 3. This supports the validity of the guidelineabout optimization of the light extraction structure obtained from thesimulation calculation. FIG. 27 also plots the light output ratioobtained for Examples 2 and 5 to 7. In FIG. 27, the horizontal axisrepresents the arrangement period (unit: nm) of the recessed andprojecting structure, and the vertical axis represents the light outputratio.

As can also be seen from FIG. 27, the tendency of the light output ratiowith respect to the arrangement period of the periodic recessed andprojecting structure is nearly matched with the simulation calculationresult. Namely, FIG. 27 supports the fact that an increment of theabsolute value of the light output ratio is due to the effect of theenhanced light extraction efficiency caused by addition of the minuterecessed and projecting structure.

It should be understood that the embodiments and the examples disclosedherein are illustrative and not limitative in every respect. The scopeof the present invention is defined by the terms of the claims, ratherthan the description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

The present invention is particularly advantageously applied to asemiconductor light emitting element that emits the short-wavelengthlight.

REFERENCE SIGNS LIST

11 positive electrode, 12 p-type semiconductor layer, 13 active layer,14 negative electrode, 15 n-type semiconductor layer, 16 substrate, 16Arear surface, 21 periodic recessed and projecting structure, 22 minuterecessed and projecting structure.

1. A semiconductor light emitting element comprising: a semiconductorlayer including a light emitting layer; wherein a surface of thesemiconductor light emitting element includes a light extractionsurface; at least one of the light extraction surface and an interfacebetween two layers having different refractive indexes in thesemiconductor light emitting element includes a periodic recessed andprojecting structure having a period that exceeds a wavelength of lightextracted from the semiconductor light emitting element, and a minuterecessed and projecting structure located on a surface of the periodicrecessed and projecting structure and having an average diameter that isnot more than 0.5 times the wavelength of the light; and the wavelengthof the light is less than or equal to 350 nm.
 2. The semiconductor lightemitting element according to claim 1, wherein an arrangement pattern ofthe periodic recessed and projecting structure is a triangular latticepattern.
 3. The semiconductor light emitting element according to claim1, wherein the periodic recessed and projecting structure includes ahigh refractive index material portion having a refractive index higherthan that of air; a cross-sectional area of the high refractive indexmaterial portion at a surface perpendicular to a direction extendingtoward the light extraction surface from the light emitting layerbecomes smaller with increasing distance from the light emitting layer;the high refractive index material portion includes a projection made ofa high refractive index material having a refractive index higher thanthat of air; and the projection has a conical shape or a semi-ellipticalspherical shape.
 4. The semiconductor light emitting element accordingto claim 1, wherein the periodic recessed and projecting structureincludes projections each having a conical shape or a semi-ellipticalspherical shape.
 5. The semiconductor light emitting element accordingto claim 1, wherein the light emitting layer includes a group-IIInitride semiconductor; and the semiconductor layer includes: an n-typegroup-III nitride semiconductor layer having an n-type conductivity; anda p-type group-III nitride semiconductor layer located opposite to then-type group-III nitride semiconductor layer as seen from the lightemitting layer and having a p-type conductivity.
 6. The semiconductorlight emitting element according to claim 1, further comprising atransparent substrate arranged on the light extraction surface side asseen from the light emitting layer and having transparency to the lightemitted from the light emitting layer.
 7. The semiconductor lightemitting element according to claim 6, wherein the transparent substrateis an aluminum nitride substrate.
 8. The semiconductor light emittingelement according to claim 1, wherein the minute recessed and projectingstructure is more densely populated on recessed portions of the periodicrecessed and projecting structure than on projecting portions of theperiodic recessed and projecting structure.
 9. The semiconductor lightemitting element according to claim 1, wherein projecting portions ofthe minute recessed and projecting structure are provided on projectingportions of the periodic recessed and projecting structure; and theprojecting portions of the minute recessed and projecting structure areprovided on recessed portions of the periodic recessed and projectingstructure.
 10. The semiconductor light emitting element according toclaim 1, wherein the period is equal to or more than 1.13 times thewavelength of the light.
 11. The semiconductor light emitting elementaccording to claim 1, wherein the period is equal to or more than 1.50times the wavelength of the light.
 12. The semiconductor light emittingelement according to claim 1, wherein the period exceeds two times thewavelength of the light.
 13. The semiconductor light emitting elementaccording to claim 1, wherein the period is equal to or more than 2.26times the wavelength of the light.
 14. A method for manufacturing asemiconductor light emitting element, comprising: preparing an elementmember that forms a semiconductor light emitting element including asemiconductor layer including a light emitting layer; forming a masklayer including a pattern on a region of the element member that forms alight extraction surface of the semiconductor light emitting element;and forming a periodic recessed and projecting structure by partiallyremoving the region that forms the light extraction surface by etchingusing the mask layer as a mask; wherein the mask layer is a metal masklayer; in the forming a periodic recessed and projecting structure, theperiodic recessed and projecting structure is formed and a minuterecessed and projecting structure is formed on a surface of the periodicrecessed and projecting structure, by performing dry etching using anetching gas; the periodic recessed and projecting structure has a periodthat exceeds 0.5 times a wavelength of light extracted from thesemiconductor light emitting element; and the minute recessed andprojecting structure has an average diameter that is not more than 0.5times the wavelength of the light; wherein the forming the periodicrecessed and projecting structure includes: the performing dry etching;and removing the metal mask layer; the period of the periodic recessedand projecting structure is the same as a period of the metal masklayer; in the performing dry etching, a reactant of the metal mask layerand the etching gas adheres to the region that forms the lightextraction surface.
 15. The method for manufacturing a semiconductorlight emitting element according to claim 14, wherein the metal masklayer includes a nickel film.
 16. The method for manufacturing asemiconductor light emitting element according to claim 14, wherein theetching gas includes a carbon-containing fluorine-based gas.
 17. Themethod for manufacturing a semiconductor light emitting elementaccording to claim 14, wherein the minute recessed and projectingstructure includes projections each having a conical shape or asemi-elliptical spherical shape.
 18. The method for manufacturing asemiconductor light emitting element according to claim 14, wherein theetching gas includes a fluorine-based gas.
 19. The method formanufacturing a semiconductor light emitting element according to claim14, wherein, in the removing the metal mask layer, the metal mask layeris removed by acid treatment.
 20. The method for manufacturing asemiconductor light emitting element according to claim 14, wherein theperiod of the periodic recessed and projecting structure exceeds thewavelength of the light.